Multicolor hardcopy images for viewing by reflection in ambient white light are produced primarily by a subtractive color process, which produces a composite image with three overlaid images of the subtractive primary colors (yellow, magenta and cyan). Subtractive color image forming processes may include printing processes and photographic processes.
Photographic processes generate yellow, magenta and cyan component images using a specially prepared, radiation-sensitive substrate, which contains precursors to the colorants. Most commonly, the three colorant precursors are coated in three different layers of a multilayer construction. Each colorant precursor is sensitized to a discrete range of wavelengths, such that the yellow, magenta and cyan images can be formed independently by exposure to different wavelength ranges for each of the three colors.
Many image-forming chemistries are known that can be used to form a permanent color image by means of image-wise exposure to radiation. For example, silver halide crystals (known as silver halide emulsions) may be sensitized to wavelengths from about 350 nm to about 1300 nm. Variations in the chemistry and processing conditions of the silver halide material may lead to either negative acting processes, by which a dye image is formed in those areas that have been exposed to radiation, or to positive acting processes, by which a dye image is formed in those areas that have not been exposed to radiation. In addition, many non-silver, full-color imaging materials have been described, based upon photogeneration or photobleaching of yellow, magenta and cyan colorants in an integral construction. In all of these cases, a full color image is generated in an integral construction using radiation of at least three different wavelengths to form the yellow, magenta and cyan images.
Analog photographic processes customarily use visible radiation of complementary color to render the image in a particular color-forming layer. Digital photography separates the image capture process from the final image generation process. Any wavelength, including non-visible infrared or UV wavelengths may, in principle, be used to generate any one of the yellow, magenta or cyan images to reproduce a full color image from digitally stored information.
Printing processes imagewise transfer yellow, magenta and cyan colorants to a substrate, typically a white substrate such as paper. Printing processes include, for example, lithographic, gravure, letterpress, and flexographic printing, as well as electrophotographic, inkjet, and other methods. It is common practice, in some of these printing processes, to add a fourth image component containing a black colorant.
Printing processes generally require that color proofs be generated at various stages in the printing process. In a typical color print process, one or more of the following proofing steps would be performed depending on the type of print job, printing budget, time requirements, and available equipment.
First, a layout proof is printed from a layout/creation computer software program, usually to an inexpensive inkjet or a monochrome laser. Second, a color layout proof is created with more accurate colors, usually utilizing a printer with some level of color calibration. An analog contract proof is then produced by utilizing a film, which also makes the printing plates, and often the print stock to be used in the final printing. The resulting proofs are used by the press operators in the final printing.
One widely used form of creating full-color analog proofs is commercially available under the trade designation MATCHPRINT (Imation Corp., Oakdale, Minn.). This system requires preparation of separate color films, which are exposed separately and then laminated together. Careful alignment of the separate color films, prior to lamination, is required to produce accurate color proofs. A properly prepared proof has a color range that is roughly equivalent to or exceeds what can be replicated on a press. A digital contract proof may also be generated from a printout of a computer file by printing using the same types of printers used in the earlier color layout proof. However, a digital contract proof must accurately mimic the color/output characteristics of the press, rather than calibrating output to match the designer's monitor or submitted color samples.
Digital proofing is gaining acceptance as digital presses and CTP schemes, neither of which require creation of film, become more prevalent. Although a digital contract proof can be generated from a file printout using a mid-level color laser or inexpensive inkjet printer, high quality digital proofing generally utilizes print methods, such as dye sublimation (dye-sub), thermal wax, and high-end inkjet printers.
Dye-sub printers are capable of producing continuous tone photographs and can easily surpass the color capabilities of the press. However, dye-sub printers have relatively high consumable costs and need to use special coated stock (usually a glossy, bright paper), malting it impossible to duplicate the actual press stock.
Finally, a press check is performed to generate a proof that is actually produced on the press. Since only relatively minor color adjustments can be made on the press, content editing and major color changes almost always require new printing plates to be created. Each proofing stage may include multiple cycles of proofing, reviewing, revising, and re-proofing.